FIG. 1 shows a known loudspeaker unit which is rectangular in shape, and shows two cross sectional views. The loudspeaker 1 is essentially centrally symmetrical in design, and has a central vertical axis 2.
The loudspeaker has a pot-shaped housing 3, which is made of metal and which has a base wall 4 and a side wall 5.
The housing 3 supports a permanent magnet 6 such as NdFeB, and a top metal plate 7. The voice coil 8 surrounds the magnet 6 and is suspended from the plastic membrane 9.
An outer plastic basket 10 supports the membrane 9, and a metal cover 11 is provided over the membrane. The cover 11 has sound transmission openings.
The lower cross section shows the electrical contacts 12 to the voice coil.
The unit 3 can have a height of 1-5 mm, and the length and width of the unit 3 in the direction running perpendicular to the direction of axis 2 can for example each be in the range 5-20 mm. These dimensions are simply to give an idea of the scale and are in no way limiting. The loudspeaker 1 is for example provided for use in a mobile telephone or in similar small scale telecommunications apparatus.
The membrane 9 is essentially U-shaped in its peripheral area. The membrane 9 has an intermediate area adjacent to the U-shaped peripheral area to which the voice coil 8 is connected. A central area is for sound generation. The membrane 9 as a whole is arranged to be oscillatory with movement in the direction of axis 2.
The magnetic field generated by the voice coil 8 interacts with the magnet 6, wherein an electrical signal representing a signal to be reproduced acoustically is sent to the voice coil 8, as a result of which the voice coil is caused to oscillate, which results in the membrane 9 bringing about the signal to be reproduced acoustically.
The electrical drive signal can be amplified and, if applicable, its signal waveform can be influenced, before being sent to the voice coil 8. An integrated circuit for this purpose can optionally be provided as part of the design.
The acoustic outlet ports which in this example are in the top surface of the cover 11 expose at least some of the speaker components to the environment. The components can be sensitive to moisture, to dust and to other contaminants.
In particular, magnetic particles are attracted by the permanent magnet 6. These particles can come from any debris (for example from keys in the pocket or also from the dust that occurs from machining metal in a workshop). Especially these magnetic dust particles create serious problems for the function of the device. The particles can be smaller than the opening size of physical filters. If enough of these small particles are attracted to the membrane by the magnet (on the opposite side), the movement of the membrane will be influenced or it can even become blocked. If the vent openings are on the other side of the membrane (i.e. on the voice coil side), then particles can influence the voice coil movement.
These problems can arise after the customer has started using a perfectly functioning device, and can thus shorten the lifespan of the device in which the loudspeaker is incorporated.
The invention is applicable to this loudspeaker design, but it is applicable more generally to loudspeakers, and is directed generally to the problem of contamination of the internal components through the sound outlet port.